Not Applicable.
Not Applicable.
1. Field of the Invention
This invention relates to an improved gas sensor and its method of manufacture, and more particularly to a gas sensor having a protective member to restrict the flow of gas into the sensor.
2. Brief Description of the Invention Background
FIG. 1 illustrates a typical micro fuel cell oxygen sensor 110 such as, for example, Teledyne Analytical Instruments"" B2 sold by Teledyne Electronic Technologies, City of Industry, Calif. The oxygen sensor 110 consists of a cathode 102 and an anode 104 sealed in a housing 106 filled with appropriate electrolyte solution. Oxygen diffuses into the interior of the sensor housing 106 through a thin sensing membrane 108. A flexible expansion membrane 112 at the opposite end of the sensor 110 permits expansion or contraction of the electrolyte volume. The sensing membrane 108 is sealed in place by means of press fit. The expansion membrane 112 is sealed in place by heat seal. Reduction of oxygen at the cathode 102 causes current to flow from the cathode 102 to the anode 104 through an externally connected sensing circuit (not shown).
Present demands on gas analyzers require that the gas sensors that they employ, such as the B2 model described above, effectively measure very low concentrations of gas. For example, it is typically the case that oxygen gas must be measured in the parts per million range. Accordingly, the gas sensor must be extremely sensitive to very low level gas concentrations.
Due to its extreme sensitivity, the gas sensor and, particularly, the cathode and sensing membrane, must be protected from high ambient gas levels during transport. This is so because exposure of the sensor to, for example, high levels of oxygen for an extended period, results in the electrolyte accumulating excessive oxygen levels. These excessive levels inhibit accurate measurement of gas at low levels due to an error signal associated with the level of dissolved oxygen in the electrolyte. To reduce the effect of this exposure, it is known to seal the sensor for shipment in double protective packaging 120 such as, for example, sealable, metalized plastic bags, having an inert (e.g. oxygen-free) environment (FIG. 2).
The packaging provides acceptable protection from, for example, ambient oxygen levels, while the gas sensor is being shipped from the manufacturing facility to the buyer. As illustrated in FIGS. 3-5, when the gas sensor is received and ready for use, the gas sensor 110 is removed from the zero oxygen packaging 120 for positioning between a cap portion 125 and a cavity portion 130 of the cell block for installation into the analyzer 130 (FIG. 5). The cell block exposes the sensor to only the sample gas stream, and provides hermetic electrical connections to the cathode, anode, and other necessary electrical terminals of the sensor. During the transition period from the zero oxygen packaging to the analyzer, however, the gas sensor is exposed to ambient air and is, thus, flooded with, for example, high levels of oxygen (typically present in ambient air in an amount of about 21 percent on a volume basis). This exposure introduces large amounts of oxygen into the electrolyte which must slowly diffuse from the electrolyte before the gas sensor can accurately and precisely measure extremely low levels of oxygen. The time required for the high ambient gas levels to diffuse from the sensor is known as the recovery time, and is typically on the order of several hours. During this time, the user typically either must vent the product being qualified by the analyzer to atmosphere, switch to a liquid backup supply, or utilize another inline analyzer that is capable of immediately measuring the gas stream with the required accuracy. All of these options have relatively high associated costs.
Existing gas sensors, such as the B2 sensor 110 of FIG. 1, have little or no ability to prevent the flood of gas, particularly oxygen, into the gas sensor during the transition period. Accordingly, gas analysis through the gas sensor is delayed for at least several hours in order for the oxygen gas to diffuse from the electrolyte, and for the gas levels inside the gas sensor to normalize. As with other prior art sensors, this delay inhibits the effective use of the analyzer to monitor the gaseous stream at the early stages of gas analysis, and may reduce the active life of the sensor.
Accordingly, the need exists for an improved gas sensor that limits the exposure of the sensor to gas, particularly to high concentrations of oxygen in ambient air, that may flood the sensor during the transition period, first beginning when the gas sensor is removed from the protective packaging until installation into the gas analyzer. In so doing, the recovery time needed after installation of the sensor into the analyzer would be greatly reduced.
The present invention addresses the above-mentioned needs by providing a gas sensor comprising a housing including a cavity. The housing defines an opening into the cavity over which is sealably positioned a gas flow resistant member that engages the housing.
The present invention is also directed to a gas sensor comprising a housing including a cavity. The housing includes a first end defining an opening into the cavity. A first electrode is positioned within the cavity, and a second electrode is positioned at the opening. A sensing membrane is positioned at the opening adjacent to the second electrode, and a gas resistant member engages the first end of the housing and is sealably positioned over the opening.
The present invention is additionally directed to a gas sensor comprising a housing including a cavity. The housing defines an opening into the cavity. A means for restricting the flow of a gas into the opening engages the housing.
Furthermore, the present invention is directed to a fuel cell comprising a gas sensor constructed according to the present invention.
Moreover, the present invention is directed to a cell block comprising a cavity portion and a cap portion that are removably secured together. One of the cavity portion and the cap portion further includes a piercing member.
The present invention is also directed to a system for sensing gas comprising a gas analyzer, a cell block and a gas sensor. The analyzer defines a cavity and is sized to receive the cell block. The gas sensor is constructed according to the present invention.
A method of inhibiting the flow of gas into a gas sensor is also disclosed herein. The method includes securing a gas flow resistant member to a housing of the gas sensor, the housing including a cavity. The housing defines an opening into the cavity such that the gas flow resistant member is sealably positioned over the opening.
The present invention is also directed to a method of performing gas analysis on a gaseous stream. The method comprises inserting a gas sensor into a cell block. The gas sensor includes a housing having a cavity therein. The housing defines an opening into the sensor cavity that is sealed by a gas flow resistant member to restrict a flow of gas into the housing cavity from a region exterior to the housing cavity. The method further comprises providing fluid communication between the exterior region of the sensor cavity and the sensor cavity, inserting a cell block into the analyzer cavity, and performing gas analysis on the gaseous stream.